1. Field of the Invention
The present application relates to a GaN system semiconductor light emitting device comprising a light emission layer, consisting of a GaN system semiconductor, and more specifically to a GaN system semiconductor light emitting device excellent in light emission efficiency and light extracting efficiency.
2. Description of the Related Art
Since a GaN system semiconductor light emitting device comprising GaN system semiconductor layers makes it possible to realize a white light LED, there is taken into account a possibility of serviceability as an electronic device for lighting applicable to a back light. The GaN system semiconductor light emitting device has as its basic structure a pn junction diode, wherein a light emission layer is interposed between an n type GaN system semiconductor layer and a p type GaN system semiconductor layer. Toward the light emission layer electrons are injected from the n type GaN system semiconductor layer whereas holes are injected from the p type GaN system semiconductor layer, whereupon they are recombined in the light emission layer to emit a light. However, especially based upon a high resistivity (several Ω·cm) of the p type GaN system semiconductor layer, only attaching directly a metal electrode for a bonding pad for supplying an electric current, to the n type GaN system semiconductor layer or the p type GaN system semiconductor layer, would inconveniently lead to a limited flow of an electric current solely around the metal electrode. As a result, in a light emission layer, which is spaced apart from the metal electrode periphery, an emission weakens. On the other hand, any light emitted around the metal electrode is prevented by the metal electrode from being extracted out of the semiconductor light emitting device.
Conventionally, in order to attain a uniform distribution of the electric current, it has been conceived to form a metal thin film for the electric current diffusion. As such the metal thin film material like a Ni/Au laminating electrode, Pt, and etc, should be annealed in an oxygen atmosphere to make it more transparent. However, although resistivity of Ni/Au or Pt is small, its transparency is not sufficient, resulting in degrading of any efficiency of taking out the emitted light or any light extracting efficiency or light exit efficiency of same. Thinning the metal film in order to increase a light transmittance, would result in the liability of easier break down of a film portion, whose film thickness is smaller than that of other film portions due to the electric current, whereupon furthermore any increase of the resistance in the lateral direction would lead to the failure to accomplish a primary object to achieve a uniform distribution of the electric current.
Moreover, there has also been proposed a method of forming an ITO (Indium Tin Oxide) electrode film for the electric current diffusion. In FIG. 6 an example embodiment has been shown, according to which there is formed the ITO electrode film for a GaN system semiconductor light emitting device. In FIG. 6, 51 designates a metal electrode, 52 designates an ITO electrode film, 53 designates a p type GaN system semiconductor layer, 54 designates a light emission layer, 55 designates an n type GaN system semiconductor layer, 56 designates a metal electrode, 57 designates a sapphire substrate. In FIG. 6, a light emission takes place in the light emission layer 54 interposed between the n type GaN system semiconductor layer 55 on the sapphire substrate 57 and the p type GaN system semiconductor layer 53. The electric current supply to the n type GaN system semiconductor layer 55 is carried out via the metal electrode 56. The electric current supply to the p type GaN system semiconductor layer 53 is carried out via the metal electrode 51 and the ITO electrode film 52.
However, although there is provided a high light transmittance of emitted light by the ITO electrode film, there occurs a formation of a Schottky type contact between the ITO electrode film and the p type GaN system semiconductor layer, thus resulting in not uniform flow of any electric current. Generally, in case of making a contact between an ITO electrode film and a n type GaN system semiconductor layer or a p type GaN system semiconductor layer, an ohmic contact is difficult to provide, while a Schottky type contact would be forced to be formed. When the Schottky type contact being formed, a potential barrier is produced between the semiconductor layer and the ITO electrode film, whereby, the driving voltage increases, thereby resulting in an increase of power dissipation and an increase of heat generation.
Moreover, there have also been made attempts to attain an ohmic property by means of a transparent electrode comprising ZnO as an electrode. This utilizes the recognition, or the fact that an ohmic contact can be obtained by providing a contact between ZnO and an n type GaN system semiconductor layer or a p type GaN system semiconductor layer.
In the present application, materials are represented by means of the symbol of elements, for example, Ga represents gallium and B represents boron.